I/O buffering problem

pwr has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I'm currently writing a perl-wrapper for Debians aptitude. The wrapper should just color some parts of the output. If I want to install a package, aptitude usually asks 'Do you want to continue? [Y/n/?]' without appending a \n after the question. The problem is, that my wrapper does not display this question until I enter something at the keyboard. This looks as follows:

...
Need to get 667kB of archives. After unpacking 3965kB will be used.
y
Do you want to continue? [Y/n/?] Writing extended state information...
...
[download]

The actual question is printed after my input. Currently, the executed code in the wrapper is very simple:

print $_ while (<STDIN>);
[download]

To get the whole thing running, I pipe the output of aptitude into the wrapper: sudo aptitude install xyz | ./wrap.pl After playing around for a while now, I know that's a problem of buffering but I'm not sure how to bypass it.

Thanks

Comment on I/O buffering problem Select or Download Code

Replies are listed 'Best First'.
Re: I/O buffering problem by pc88mxer (Vicar) on Feb 15, 2008 at 20:17 UTC
When you use `<STDIN>`, perl waits for the new line before returning the result. You need to use `read` or `sysread` to get data as soon as it is available.	[reply] [d/l] [select]
Re^2: I/O buffering problem by pwr (Novice) on Feb 15, 2008 at 20:26 UTC
Ah, that perfectly solves my problem, thank you!	[reply]
Re: I/O buffering problem by hipowls (Curate) on Feb 15, 2008 at 20:22 UTC
You output is buffered. To turn it off set $\| to a true value, for example `$\| = 1` Update: I answered the wrong question. I thought the OP was printing the prompt, he is trying to read it. `read` or `sysread` is the correct answer as given by pc88mxer	[reply] [d/l] [select]